In the preparation of various substrates, techniques have been developed to increase the strength of the substrate by insolubilizing materials onto the substrate to reinforce the substrate. One example of such a substrate is paper. In the production of paper, cellulose has been pulped in a slurry, and the slurry applied to a screen to orient the fibers and drain away liquids. Typically from the screen the paper is then squeezed between rollers to further reduce the water to produce a sheet of paper after suitable drying. The strength of the paper produced is dependent upon the nature of the pulp. For example, Virgin pulp of a given quality typically produces a much stronger paper than pulp from repulped paper of the same quality. Also, the strength achievable from the pulp decreases with each recycling evolution. Thus, there has always been an interest in additives which could increase the strength of the paper. Strength of the paper heretofore has been increased either by use of a size press operation or by wet end addition. The main concern is to eliminate size press additions because it is extremely inefficient from both a production and energy content standpoint. Typically, size press additions have been conducted by wetting the finished paper of flooded rollers with a starch solution to soak the starch into the paper subsequent drying thereby increasing the strength of the paper. This procedure suffers several drawbacks, among of them being, there is a limited amount of starch which can be added in this manner, thereby limiting the strength increase possible since the sheet can only absorb a fixed amount of solution. Furthermore, size presses are a large capital item requiring large amounts of space when coupled with the extra dryers required to redry the sheet.
Paper consumption has increased worldwide and is expected to increase further. A large portion of solid waste generated is paper and paper products. Many nations have undertaken efforts to reduce all types of waste products including paper in order to conserve landfill space. As a result, there is increasing interest and desire to recycle paper products. One disadvantage and limitation heretofore on the recycling of paper was the inability to achieve the desired strength of paper made in whole or in part from recycled pulp. The difficulties in achieving sufficient strength will become magnified as it is attempted to recycle paper which has already been recycled one or more times. Indeed in Europe where recycling is more intensive than in the United States it has become commonplace practice to make certain grades of paper entirely from recycled fiber especially in the boxboard grades.
The products are made from recycled boxes and mixed waste without the use of any virgin fiber and for certain grades e.g. fluting grade paper it is impossible to make the quality standard required without a large increase in the stiffness parameter as measured by the concorra medium test (CMT) or latterly the STFI test. To effect such improvements as is necessary, the mills have been forced to add starches by means of a size press in amounts varying between 3% and 10% based on fiber. To date it has been generally accepted that wet end addition of conventional reinforcing agents such as starches, carboxymethyl cellulose, polyvinyl alcohol etc., in various states of charge density or charge sign have failed to produce the quality improvements equivalent to a size press addition of say 4-10% starch for several reasons. These are:
1. The cost of the wet end additives has been excessive when compared to the total cost of size press addition of raw starch. PA0 2. Technically, the technology of wet end addition has been incapable of adding sufficient reinforcing agent to the pulp such that it is retained in the correct fashion and provides the type of strength required. PA0 (a) Cationization of the polymer to allow increase substantivity to the anionic sites of the substrates eg; cationic starch. PA0 (b) Insolubilization of certain anionic species with polyvalent metal ions such as aluminum, iron, zirconium eg; carboxymethyl cellulose. PA0 i) Reinforcement of paper to provide higher strength properties and possibly sizing by addition at the wet end of the paper machine. PA0 ii) Provide a binder system with significant wet strength. PA0 iii) Earth conditioning--production of the gel in any dust like substance produces a dust free wet solid that will retain moisture and probably act as a growth sustaining medium and consequently be usable in a situation such as land remediation where sand and/or any other light dusting soil needs to be stabilized for sufficiently long as to allow growth of ground cover vegetation. The product also stops dusting. PA0 iv) Since a distinct gel can be made it is possible to extrude the product into fiber or other shapes which on drying will have quite good tensile properties. In the case of fibers it is probable that a matrix could be spun possessing super absorbent properties which could be used in domestic products such as diapers etc. It is also possible that production of a fiber after sufficient drying might produce a textile fiber possibly after exposure to a cross-linking agent such as formaldehyde or glyoxal. PA0 v) With respect to (iv) above another application would be in the production of wound dressings because of the super absorbent quality. PA0 vi) The use of high levels of the gel with respect to fiber produces very stiff board/wood like materials and it is envisaged that products such as medium density fiber board, hard board, or any other type of bound matrix board where conventionally phenolic resins or the like are used as binders could be replaced by this system. The advantages are economic, no volatile solvents, and the product can be wet formed without the production of a polluted effluent. This could offer significant advantages over present production techniques. PA0 vii) We have found that as little as 1-3% of binder system in sand produces an extrudable/curable resin that binds the sand such that the sand can be used as a mold for such applications as foundry casting sand. The dried sand appears to break up in water after use producing non of the toxic pollutants associated with phenolic resins. Again there are no VOC's present at the formation stage and the whole reaction can be carried out wet and the molds simply oven dried. PA0 viii) Dust suppression in utilities of all kinds such as red mud, coal dust processing or any other inherently dirty/dust process. The cake produced by reaction with the gel is water resistant and thus could be exposed to rain and wind. PA0 ix) When one solution is layered on top of the other a very thin film of the gel is formed. This will probably have semi-permeable qualities and could be used in some specialists separation technologies using osmosis etc. PA0 x) Ceiling/Wall boards can be made from mixtures of the gel with alumino silicate or pulp fibers using high percentages of the starch. These products would have high thermal resistance and could replace conventional lagging materials. Additionally, the system could be used as a binder to produce lagging products. PA0 xi) Modified starches can also be used to produce different but anticipated effects by the inclusion of hydrophobes on the backbone. One such application is the removal and passication of stickies in paper making systems. We have discovered that if dispersed sticky materials such as ethyl vinyl acetate are dispersed in water to provide a cloudy dispersion, after contact with acetylated starches which have anionic functionality, if a flocculent is added to the same mixture a flocculation takes place which removes the turbidity associated with the sticky. Treatment of the raw emulsified sticky with the same flocculent at the same dose rate does not produce a clarification reaction. We believe that any hydrophobically substituted starch which has anionic sites can be similarly reacted to remove stickies as during the paper making process flocculents are added to the stock which would cause their retention. PA0 xii) Any other type of board such as laminate equivalents for use in such articles as doors or any other decorative finish. PA0 xiii) With one shot wet molding technology it is envisaged boxes could be made without the formation of paper.
Heretofore, the paper maker has tended to use wet end additions (i.e. chemicals that are applied in the pulp slurry) where possible to achieve the relevant increase in strength of a particular grade when using a recycled substitute furnish for the original virgin grade. There are many technical limitations on the use of wet end additives to achieve strength e.g. cationic starch, polyvinyl alcohol, xanthan gum etc. Generally, it is the presence of tramp materials usually referred to as "anionic trash" that inhibit the performance of these additives such that the strength improvements achieved (if any) are not economically viable. As an alternative the paper maker has resorted to size press technology to gain large increases in strength as the uptake is quantitative based upon saturation of the finished sheet with a known strength solution of reinforcing agent. The results obtained by this technology are recognised by the industry as superior to wet end addition especially in the area of the promotion of stiffness, a much valued property in many grades and especially so in boxboard.
However, it is recognised that size press additions are dry end additions and suffer large economic penalties. Thus there has been a continuing need to eliminate the size press operation to increase efficiency and decrease cost if the same quality improvements are available by wet end addition.
The prior methods for production of paper sought to improve the strength by the addition of polyhydroxylated polymers such as starch, polyvinyl alcohol, carboxymethyl cellulose, xanthan gum, guar gum and other such natural or synthetic binding agents which act by hydrogen bonding to the substrate. However in previous processes it has not been possible to fix from solution the charge neutral versions of these polymers into the wet formed sheet as they remain in solution and simply drain through the paper being produced. Several processes have been patented and demonstrated as methods for the attachment of these polymers to the substrate. The generally accepted methods are:
These processes allow incorporation of the additive into the wet end of the paper making process thus avoiding the expensive extra process of size press addition which involves extra drying of the sheet from the rewet needed. However, these processes are subject to several limitations based on the quality of the fiber, anionic trash, pH etc., and had limited ability to increase the strength of the paper especially with respect to stiffness. The process of the present invention allows for a greatly improved performance especially in "dirty systems" operating at neutral pH's and allows for greatly improved strength by the addition of appropriate entities to the wet end of the paper making process over strengths availably by prior processes. The present invention in many applications, will allow the elimination of the size press resulting in considerable cost savings and process simplification. The advantages of the present invention include: (a) ability to increase strength substantially over prior methods, (b) the elimination of sizing equipment, (c) the reduction of refining, (d) the ability to select reactants, reaction conditions to achieve desired strength increases or other desirable properties in relation to characteristics of the pulp being utilized and (e) improved drainage equivalent to or greater than generally accepted micro particle technologies.
Since during the reaction of the components in the process is an almost quantitative production of gel like precipitate can be formed, (the physical properties of which varied with starting material).
The process of the present invention would have application in the following areas:
In addition to the specific applications referred to above, it is also considered possible to utilize a cationic high charge starch material having a long chain backbone, and to mix same with an anionic starch to cause the required precipitation. It is believed that such application would be particularly advantageous in connection with certain third world countries who are able to mix the two identified starches together and produce a product, which could be used for forming board material and the like.
It would thus be seen that the process of the present invention has numerous applications and the mixing of the two integers produces a product which has high strength and can be used in a number of applications.
The strength characteristics of paper are evaluated in many different ways. There is strength to be measured in the machine direction, cross machine direction or thickness direction. Various types of strength measurements are burst, tear, tensile, stiffness, taber stiffness, ring crush, fold endurance, concorra medium test (CMT) and STIFI. These physical properties (amongst others) can be measured using standards of the Technical Association of Paper Processing Industry (TAPPI). The present invention has the additional advantage that by varying the amount of reactants it is possible to effect changes in the different types of strength, thus, the present invention has the advantage of the flexibility to selectively affect various types of strength.